1. Field of The Invention
The present invention relates generally to a preformed piece for an outer or inner race of a bearing, method for manufacturing same, and a mandrel and a roll used therein. More specifically, the invention relates to the shape of a forming die comprising a mandrel and a roll, which is used for forming a preformed piece for an outer or inner race of a bearing having a bearing ring, using the cold rolling working.
2. Description of The Prior Art
In a conventional method for manufacturing an outer or inner race of a bearing, a steel blank is hot-forged to obtain an annular blank. Then, after the annular blank is annealed and shot-blasted, it is cold-rolled to form a preformed piece 1 for a bearing outer race (which will be hereinafter referred to as "a bearing outer-race preformed piece") or a preformed piece 14 for a bearing inner race (which will be hereinafter referred to as a "bearing inner-race preformed piece"), as illustrated in FIGS. 6 and 7. The term "preformed piece" as used herein means an object that has been subjected to preliminary incomplete shaping before undergoing complete or final processing. The preformed piece thus obtained is turned by means of a lathe to produce a bearing outer-race product or a bearing inner-race product of a product shape 6 or 19 illustrated by a two-dot chain line in FIGS. 6 and 7. In the cold rolling working, an inner raceway groove 7 or an outer raceway groove 20, which has a circular curved surface, is provided on an inner surface 2a of the bearing outer race or an outer surface 15b of the bearing inner race to form the bearing outer-race preformed piece 1 or the bearing inner-race preformed piece 14.
In the case of the bearing inner-race preformed piece 14 having the raceway groove on the outer surface 15b thereof, the core run out is easy to occur since the bearing inner-race preformed piece 14 is held by at tightening chuck when it is turned to form a final product. For that reason, an unbalanced load is easy to be applied thereto, so that the product yield is poor. Therefore, the number of the bearing inner-race preformed piece 14 having the raceway groove is less than the number of the bearing outer-race preformed piece 1 having the raceway groove. In addition, the forming-die surface of a forming die of a mandrel or a roll used for forming these bearing preformed pieces using the cold rolling working, has a bearing ring shape, which is obtained by adding a turning stock y of 1.5 mm shown in the drawing to the turned surface of the bearing ring of the bearing outer-race product or the bearing inner-race product, in order to facilitate the turning working.
In the turning of the bearing preformed piece formed by the mandrel or roll, when a turning tool for finishing having a turned curved-surface cuts into the bearing outer-race product or the bearing inner-race product to the turned curved-surface of the bearing ring, elongated thin chips are produced particularly at the beginning of turning. In addition, when the center of the bearing ring is offset, the direction for discharging the chips can not be determined, so that the chips enter a gap between the working face and knock-out face of a collet chuck which holds the preformed piece. Thus, the nipped chips may get into trouble such as the production of defective products. In addition, the chipping of the turning tool for finishing may occur due to the unbalanced load, so that the turning tool for finishing must be used while being rotated by three or four surfaces, thereby decreasing the life time of the tool.